THE ESSENTIALS OF ECONOMICAL MACHINING 203 pieces, it is not permissible to clamp directly on their upper surfaces. Lateral pressure is adopted in such cases, and also for those where the upper surfaces have to be machined all over. The clamps are better if set diagonally (fig. 35) to exer- cise a downward pressure. Often it is necessary to set a stop against one end of the work in the line of direction of the cutting-tool in order to prevent the occur- rence of slip by reason of the pressure in the longi- tudinal direction. For hold- ing substantial pieces, direct clamping is adopted. But, since tall bolts are apt to be unstable under the stress of heavy cutting, advantage is taken of the presence of suitable lower sections on which to bed the clamps. These may be stout flanges, bosses, bores, or recessed portions, to be utilized by the judgment of the machinist. Cylindrical and Bored Work.—This is located and held in vee blocks, which ensure parallelism of the work with the table. Parallel shafts Work Fig. 36.—Large Vee Block and Vee'd Clamp Sectional Elev Fig. 37.-—Vee Blocks for Two Shafts, Clamp Plates adjusted with Grub-screws and thrown off with Springs S, Throw-offSprings Gt Grub Screws for Adjust- ment of Clamfs and tubular portions of castings when located in vees of equal heights will lie parallel with the table. If shafts are put through holes, and laid in vees, the holes will be parallel. The clamps are variously set, according to outlines, dimensions, and the avoidance of portions to be machined.